Mass-produced products are generally processed in a special-purpose processing line in which multiple special-purpose processing machines, each of which is specialized to process a single component, are provided. In the special-purpose processing line, when the design of the component to be manufactured is changed, it may be required to replace any of the special-purpose processing machines to another one. On the other hand, in order to accommodate a diversification of products and a change in market needs, a mixed processing line that can simultaneously process different components has been widely used.
In the mixed processing line, not only the special-purpose processing machines but also general-purpose machining centers are provided, so as to increase flexibility of the processing line. Recently, some mixed processing lines may be comprised of only the general-purpose machining centers. Various processes may be performed by the general-purpose machining center. Japanese Unexamined Utility Model Publication (Kokai) No. 61-50622 (JP-U-61-50622) discloses, as a technique for increasing the processing flexibility in the machining center, a tap holder that is mounted on a spindle of the machining center so as to enable a tapping process.
However, in comparison with a special-purpose tapping machine equipped with a multi-axis head, the tap holder disclosed in JP-U-61-50622 cannot obtain higher processing efficiency. In this regard, considering the fact that the rotation speed and feed speed of the spindle in the special-purpose processing machine with the multi-axis head are higher than those of the common general-purpose machining center, it may be expected that the processing efficiency comparable to that of the special-purpose processing machine can be obtained by increasing the rotation speed of the spindle in the general-purpose machining center. For example, some recent heavy-duty (high-speed rotation and high-speed feed) drills can reach a cutting speed of 350 m/min and a feed speed of 1 mm/rotation.
However, when the rotation speed of the spindle in the general-purpose machining center is increased, a problem may arise in that a longer start-up time for the operation of the spindle is required. Thus, even though the processing speed is increased by increasing the rotation speed of the spindle, the start-up time becomes longer, and as a result, processing efficiency cannot be improved very much. In particular, in a tapping process, it is necessary to reverse the rotation of the spindle in order to withdraw a tapping tool from a workpiece, and thus to frequently switch the rotational direction of the spindle between the forward and reverse directions, and therefore, processing efficiency tends to be remarkably reduced.